Seismicity in Iceland 2003 - Veðurstofa Íslandshraun.vedur.is/ja/skyrslur/Jokull54skj2003.pdf · Seismicity in Iceland 2003 Gunnar B. Guðmundsson, Steinunn S. Jakobsdóttir and

Data report

Seismicity in Iceland 2003

Gunnar B. Guðmundsson, Steinunn S. Jakobsdóttir and Bergþóra S. ÞorbjarnardóttirDepartment of Geophysics, Icelandic Meteorological Office, Bústaðavegur 9, 150 Reykjavík, Iceland;

[email protected], [email protected], [email protected]

Abstract — In 2003 the SIL seismic monitoring network consisted of 41 seismic stations. The total number ofearthquakes located by the SIL system in 2003 was about 10400, which is the lowest number in the last 10 years.However the number of earthquakes of magnitude greater than1.5 was similar to 2002. Eight felt earthquakeswere reported in 2003. The largest earthquake in 2003 occurred about 7 km southwest of Kleifarvatn on theReykjanes Peninsula on August 23 at 02:00. It had a magnitudeMlw = 5 and was felt in many parts of SW–Iceland. The aftershock activity was most intense the first two days after the main shock and culminated abouta week later. Seismicity beneath the Mýrdalsjökull glacierwas high and as in 2002 was continuous throughoutthe year under the western part. From mid year 2003 the seismic activity has increased at Grímsvötn volcanoin the Vatnajökull ice cap. Some earthquake swarms occurredon the northern part of the Reykjanes Ridge withearthquakes greater than 4. Several earthquake swarms withearthquakes of magnitude about 3 were recordedin the Tjörnes Fracture Zone. Aftershock activity still continues on the main faults from June 2000 in the SouthIceland Seismic Zone.

INTRODUCTION

The SIL seismic system is a network of 3-componentdigital seismic stations and a data processing system(Jakobsdóttiret al., 2002). At the end of the year 2003the network consisted of 41 SIL seismic stations. Onenew SIL seismic station, Krókóttuvötn, was installednorth of Mývatn in the middle of the year. Two sta-tions were discontinued in January 2003, Skamma-dalshóll, south of Mýrdalsjökull, and Grindavík, onthe Reykjanes Peninsula.

A total of 10400 earthquakes were located by theSIL seismic system in 2003 compared with about14000 in 2001 and 2002 (Figures 1 and 2). The num-ber of earthquakes greater than or equal to 1.5 is sim-ilar for the years 2003 and 2002.

The magnitude scale used here is our local mo-ment magnitude scaleMlw (Slungaet al., 1984). It isscaled in a manner resembling the Richter local mag-nitude scaleMl.

Here we focus mainly on the seismic activity on theReykjanes Peninsula and beneath Mýrdalsjökull andVatnajökull, which had high seismic activity in 2003.Following is a short description of the seismicity inother areas during 2003.

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Figure 1. Annual number of earthquakes from 1994to 2003. –Árlegur fjöldi jarðskjálfta fyrir árin 1994til 2003.

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Figure 2. Earthquake epicenters (red dots) recorded in the year 2003 by the SIL seismic system. The maintectonic features of Iceland are shown. RR denotes the Reykjanes Ridge, SISZ the South Iceland SeismicZone, EVZ the Eastern Volcanic Zone, WVZ the Western Volcanic Zone, TFZ the Tjörnes Fracture Zone, Vthe Vatnajökull Ice Cap and M the Mýrdalsjökull glacier –Kort sem sýnir jarðskjálfta (rauðir punktar) semstaðsettir voru með SIL kerfinu árið 2003.

The seismic activity in the South Iceland seismiczone (SISZ) was mainly concentrated on the faults ofthe June 2000 main shocks. The number of these af-tershocks is decreasing and most of them cluster closeto the origin of the main shocks.

The Tjörnes Fracture Zone had several earthquake

swarms. Earthquake swarm activity occurred northand east of Grímsey in June, August and October. Thebiggest earthquake in these swarms, with magnitudeMlw = 3.7, occurred 18 km north of Grímsey on Oc-tober 15. There was also swarm activity in the mouthof Eyjafjörður and in Öxarfjörður.

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Increased earthquake activity has been on thenorthern part of Reykjanes Ridge since 2000. At theend of April, swarm activity was recorded close toGeirfuglasker with an earthquake of magnitude 4. Anearthquake of magnitude 4 was also recorded about 15km SW of Eldeyjarboði in a swarm at the end of theyear. An earthquakeMb = 5 (NEIC) was recordedon 62.9 degrees north, on the Reykjanes Ridge onJuly 19.

An earthquake of magnitudeMlw = 3.2 occurredbeneath Nesjavellir in the Hengill area on March 11.It was felt in Reykjavik and Selfoss. Some smallearthquakes at about 16 km depth were recorded byHeimaey in the Westmann Islands in August andNovember.

REYKJANES PENINSULA

The Reykjanes Peninsula is a continuation of theoblique spreading ridge, Reykjanes Ridge, and con-nects it to the South Iceland Seismic Zone (SISZ) andthe Western Volcanic Zone (WVZ).

On August 23, 2003 at 02:00 anMlw = 5 earth-quake occurred west of Kleifarvatn on the Reykja-nes Peninsula at about 4 km depth (Vogfjörð, 2004);(Figure 3). The earthquake was felt in many parts ofSW–Iceland. More than 1200 aftershocks followedon the same day and the day after of which 10 weregreater than 3 in magnitude. The aftershock activ-ity culminated about a week later. The epicenters ofthe aftershocks aligned on a NS-line containing themain shock and also on an ENE-line from the mainshock into Kleifarvatn. The optimum fault plane so-lution for the main event and the aftershocks indicatethat the fault plane is a N–S right–lateral strike slipfault. This seismic activity on the Reykjanes Penin-sula is the greatest since June 2000 when 3 earth-quakes of magnitude greater than 5 were triggered onthe Reykjanes Peninsula by the main shock in SISZon June 17, 2000 (Vogfjörð, 2003).

An earthquake swarm was recorded in Bláfjöllin early April with the biggest earthquake of mag-nitude 2.2. A small earthquake swarm took place atFagradalsfjall in October.

MÝRDALSJÖKULLThe Katla volcano is located under Mýrdalsjökull. Ithas shown signs of unrest since 1999 when a jökul-hlaup emerged from the southern part of the glacierand a new cauldron was formed and other cauldronsshowed changes in size (Guðmundsson et al., 2000;Vogfjörð 2002). GPS measurements in Austmanns-bunga, at the northeastern rim of the Katla caldera,have also shown uplift and outward displacementsfrom the year 2002 (Sturkellet al., 2003a). The seis-micity under Mýrdalsjökull is mainly concentrated intwo areas, within the Katla caldera and to the west ofthe caldera where the seismicity is usually seasonalwith nearly all earthquakes occurring in the secondhalf of the year (Einarsson and Brandsdóttir, 2000).In 2002 the pattern of seismicity in the western partof Mýrdalsjökull changed from seasonal to continu-ous seismic activity throughout the year (Þorbjarnar-dóttir et al., 2003). The majority of earthquakes lo-cated under Mýrdalsjökull in 2003 originated underthe western part, where the seismic activity had thesame continuous pattern as in 2002. The seismicitywithin the caldera was mainly under its northern part(Figure 4). On September 22, an earthquake swarmoccurred beneath the northeastern rim of the caldera,close to Austmannsbunga. The biggest earthquake inthat swarm had a magnitudeMlw = 3.9.

VATNAJÖKULLThe Vatnajökull ice cap includes several volcanic sys-tems. The most active is the Grímsvötn volcano,which erupted last in 1998, and has since been inflat-ing (Sturkellet al., 2003b). From mid year 2003 theseismicity at Grímsvötn increased (Figure 5). Most ofthe earthquakes have their origin close to the southernrim of the Grímsvötn caldera.

In the beginning of July 2003 a small earthquakeswarm occurred beneath Bárðarbunga. Some earth-quakes were recorded on Loki ridge, the ridge strikingeastward from Hamarinn, and also some in Kverk-fjöll. Icequakes were recorded in Skeiðarárjökullin relation to jökulhlaups from Grænalón or rainfall(Robertset al., 2005).

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Figure 3. The map shows earthquake epicenters (red crosses)and an E–W cross section of the earthquake depthdistribution on the Reykjanes Peninsula in 2003. The green star denotes the location of theMlw = 5 mainshockon August 23. The mainshock’s focal mechanism is also shown.– Kort sem sýnir jarðskjálfta (rauðir krossar)á Reykjanesskaganum árið 2003 ásamt dýptarþversniði af jarðskjálftunum. Staðsetning stærsta skjálftans, 5stig, er sýnd með grænni stjörnu. Brotlausn skjálftans og eftirskjálftar endurspegla færslur á nær lóðréttu NShægrihandar sniðgengi.

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Figure 4. Cumulative strain release (in erg1/2) and cumulative number of earthquakes in the two active areasbeneath Mýrdalsjökull, west of Goðabunga and within the Katla caldera. Magnitude vs. time 1999–2003 isplotted beneath each graph. –Línuritin sýna uppsafnaða streitu og uppsafnaðan fjölda jarðskjálfta frá 1999 til2003 í vestanverðum Mýrdalsjökli og í Kötluöskjunni, og stærð sem fall af tíma.

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Figure 5. A map of earthquake epicenters (red dots) beneath and north of Vatnajökull (top). The graph shows cu-mulative strain release and cumulative number of earthquakes within the Grímsvötn central volcano from 2001to 2003 and a magnitude vs. time plot. –Kort sem sýnir upptök jarðskjálfta (rauðir punktar) á Vatnajökuls-svæðinu. Línurit sýnir uppsafnaða streitu og uppsafnaðan fjölda jarðskjálfta í Grímsvötnum ásamt stærð semfall af tíma frá 2001 til 2003.

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Table 1. Earthquakes felt in 2003. –Jarðskjálftar sem menn fundu á árinu 2003.

Origin date Origin time Latitude Longitude Depth Mlw Felt atkm location

2003-03-11 17:27:45.1 64.109 -21.265 7.1 3.2 Selfoss,Mosfellsbær

2003-04-06 21:05:02.9 63.947 -21.321 7.5 3.2 Hveragerði,Þorlákshöfn

2003-05-15 17:34:33.9 63.860 -22.382 8.3 3.3 Grindavík2003-05-16 06:44:05.6 63.853 -22.385 8.2 2.2 Grindavík2003-05-26 17:50:42.9 65.638 -16.878 5.0 1.9 Reykjahlíð2003-08-23 02:00:11.8 63.902 -22.086 4.1 5.0 SW-Iceland2003-09-13 13:39:29.8 63.934 -21.405 5.0 2.3 Ölfus2003-11-19 23:06:04.7 63.953 -21.074 4.7 2.4 Selfoss

On October 3 and 18, two earthquakes of mag-nitude 3.2 and 3.4 occurred near Dyngjuháls closeto the northwestern edge of Vatnajökull ice cap. Anearthquake swarm lasting from November 3–22 tookplace in Herðubreiðartögl in the Askja fissure swarm.The biggest earthquakes in that swarm had magni-tudes close to 3.

Acknowledgements

This paper is based on weekly reports made by em-ployees of the Department of Geophysics, IcelandicMeteorological Office, in 2003. They are BergþóraS. Þorbjarnardóttir, Erik Sturkell, Gunnar B. Guð-mundsson, Halldór Geirsson, Hjörleifur Sveinbjörns-son, Kristín S. Vogfjörð, Matthew J. Roberts, Sig-urlaug Hjaltadóttir, Steinunn S. Jakobsdóttir, VigfúsEyjólfsson and Þórunn Skaftadóttir. They contributedto the operation of the SIL system and in the build-ing up of the earthquake database. Jósef Hólmjárnand Sighvatur K. Pálsson are responsible for main-taining the SIL seismic stations. Reynir Böðvarssonand Ragnar Slunga are the main designers of the SILsystem. All figures in this paper were made with GMTsoftware (Wessel and Smith, 1991).

ÁGRIP

Jarðskjálftavirkni á Íslandi árið 2003

Í lok árs 2003 var 41 jarðskjálftastöð í SIL jarð-skjálftamælanetinu. Ein ný SIL stöð, Krókóttuvötnnorðan við Hlíðarfjall í Mývatnssveit, bættist við SILnetið um mitt ár. Tvær jarðskjálftastöðvar voru lagð-ar niður í byrjun árs, Skammadalshóll í Mýrdal ogGrindavík á Reykjanesskaga. Um 10.400 skjálftarmældust undir og við landið á árinu. Stærsti skjálft-inn varð aðfaranótt 23. ágúst kl. 02:00 og mæld-ist staðbundin vægisstærð hansMlw = 5. Upp-tök hans voru á um 4 km dýpi vestan við Kleifar-vatn á Reykjanesskaga. Brotlausn skjálftans og eft-irskjálftar benda til þess að brotaplan hans hafi verið ánæstum lóðréttu NS hægrihandar sniðgengi. Skjálft-inn fannst víða um SV–land allt austur til Víkur íMýrdal. Skjálftahrinan var öflugust fyrstu tvo sól-arhringana en síðan dró verulega úr henni og hennivar lokið fyrir mánaðamótin. Á árinu urðu nokkrarskjálftahrinur nyrst á Reykjaneshryggnum. Stærst varhrina við Geirfuglasker í lok apríl með skjálfta yfir4 stig. Í lok ársins varð hrina sunnar á Reykjanes-hryggnum eða um 15 km suðvestan við Eldeyjarboða.Stærsti skjálftinn þar mældist um 4 stig. Undir vest-anverðum Mýrdalsjökli, vestan við Goðabungu, varviðvarandi skjálftavirkni allt árið eins og verið hef-

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ur frá miðju ári 2001. Undir Kötluöskjunni var virknimest við Austmannsbungu. Frá miðju ári 2003 hefurskjálftavirkni undir Grímsvötnum farið vaxandi. GPSmælingar sýna aukinn kvikuþrýsting undir Kötlu ogGrímsvötnum. Í Skeiðarárjökli mældust nokkrar ís-skjálftahrinur sem tengdust jökulhlaupum eða mik-illi úrkomu. Í október mældust 2 skjálftar yfir 3 aðstærð með upptök undir Dyngjuhálsi við norðvestan-verðan Vatnajökul. Í nóvember var skjálftahrina viðHerðubreiðartögl, austan við Öskju. Stærstu skjálft-arnir í þeirri hrinu mældust um 3 stig. Úti fyrir Norð-urlandi var skjálftavirkni mest úti fyrir mynni Eyja-fjarðar á Húsavíkur-Flateyjarsprungunni og einnig áGrímseyjar-Öxarfjarðar beltinu, bæði austan Gríms-eyjar og inn í Öxarfirði. Stærstu skjálftarnir á þess-um svæðum voru rúmlega 3 að stærð. Á Suðurlandivar ennþá áframhald á eftirskjálftavirkni á Holta- ogHestvatnssprungunum frá stóru skjálftunum árið 2000en virknin þar minnkar þó stöðugt. Undir Nesja-völlum við Hengil mældist þann 11. mars skjálfti aðstærð 3 sem fannst á höfuðborgarsvæðinu og Suður-landi. Tvær litlar skjálftahrinur mældust undir Heima-ey í Vestmannaeyjum í ágúst og nóvember en slíkarhrinur eru mjög óvanalegar þar.

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